Lab Activity 3: Gravimetric Stoichiometry 2

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1 Chemistry 2202 Gravimetric Stoichiometry Lab 2 1 Lab Activity 3: Gravimetric Stoichiometry 2 Introduction: Stoichiometry involves making predictions about how much of one substance should react or be produced relative to a given amount of another species involved in a reaction. The following balanced equation shows the reaction between the calcium carbonate in Tums and vinegar (acetic acid): CaCO 3(aq) + 2 CH3 COOH (aq) CO 2(g) + H2 O (l) + Ca(CH3COO) 2(aq) In this activity, you will calculate the amount of carbon dioxide gas that should be produced (theoretical yield) from a measured amount of a reactant and compare it to the amount of carbon dioxide that is actually produced (actual yield). The predicted amount is based on stoichiometric calculations, the actual amount is based on careful mass measurements. Careful measurements and attention to detail are very important in this activity. The activity will be carried out over a period of three days. You will need access to the lab for each of those days. Be sure to inform your supervisor(s) of this requirement. Textbook Readings MHR Chemistry pages x-xiii: Safety in Your Chemistry Laboratory page 143: Filtration page : Math and Chemistry WebCT See the WebCT web pages Unit 1 Section 2 Lab 3 for iluustration on use of a balance and filtration techniques. Problem How does the mass of a product relate to the mass of a reactant in a chemical reaction? Materials and Apparatus: per pair of students: 1 balance 2 250mL beaker 1 25 ml graduated cylinder 3 Tums tablets 1 bottle table vinegar 1 stirring rod 1 watch glass 1 wash bottle containing distilled water 1 filtration apparatus (ring stand, ring clamp, funnel, filter paper)

2 Chemistry 2202 Gravimetric Stoichiometry Lab 2 2 Safety 1. Wear safety glasses throughout this activity. 2. Dispose of chemicals as directed in the procedure. 3. Wash your hands at the end of each laboratory session. Procedure Day 1 1. Measure and record the mass of a 250 ml beaker. 2. Add 3 Tums tablets to the beaker and measure the mass. Record your data in the table in the Observation section of the lab. 3. Measure and record the mass of a 25 ml graduate cylinder. 4. Add 20 ml of vinegar and measure the mass. Record your data. 5. Slowly add the vinegar from the graduated cylinder to thetums in the beaker. Don't let it bubble over! Swirl the beaker gently after each addition of viengar. 6. Once the bubbling has stopped, record your observations, and the mass of the beaker and reaction mixture. Day 2 1. Record any visual observations describing the changes that have occurred in the colour of the solution, the appearance of the the Tums, and the appearance of gas bubbles. 2. Fold the filter paper in half, make a crease, fold it again to make a quarter disc, and make a second crease. 3. Find and record the mass of a piece of the filter paper. 4. Set up the filtration apparatus using the funnel, ring and ring stand, and the 500 ml beaker. 5. Let the unreacted Tums settle to the bottom of the beaker. 6. Decant (i.e. pour off slowly) the solution into the funnel using a stirring rod against the lip of the beaker in order to direct the liquid into the funnel. Keep the liquid level in the funnel at ¾ full or less to prevent liquid from by-passing the filter. 7. Use the rubber tip of the stirring rod to transfer the unreacted Tums from the beaker onto the filter paper in the funnel. 8. Use the wash bottle to rinse any remaining unreacted Tums from the beaker and into the funnel. 9. Wash the unreacted Tums several times to remove any soluble substances like acetate ions and calcium ions from the residue. 10. Remove the filter paper cone from the funnel by pressing your thumb against the side that has three layers of filter paper in a gentle upwards motion 11. Place the filter and the residue (i.e. the unreacted Tums) on a clean watch glass and unfold.

3 Chemistry 2202 Gravimetric Stoichiometry Lab Leave the paper containing the unreacted Tums in a safe place to dry for one or two days. 13. Dispose of the filtrate in the 500 ml beaker by pouring it down a sink and flushing the sink with plenty of water. Day 3 1. When the residue (i.e. the unreacted Tums) is dry, find and record the mass of the filter paper and the unreacted Tums. You should repeat the mass measurement later on in the day to ensure that it is constant. If there is a change, then the unreacted Tums is not completely dry. Let it dry overnight and repeat these steps the next day. 2. Inspect the paper and record your observations of the appearance of the dried paper. 3. Once you have made your final observations and mass measurements, dispose of the filter paper and unreacted Tums by placing it in a waste basket.

4 Chemistry 2202 Gravimetric Stoichiometry Lab 2 4 Name(s): School: page: of Purpose: Procedure/Materials Changes Describe any changes you made to the procedure here. Observations Record your observations in these tables. Note: all mass measurements must have a precision of 0.01 grams; in other words, they must show two decimal places. Table 1: Visual Observations and Mass Data Observations as reaction proceeds Appearance of reaction mixture in beaker after one day Appearance of filter paper after drying Appearance of filter paper after drying Table 2: Mass Data Mass of empty beaker: Mass of beaker +Tums Mass of empty 25 ml graduate cylinder Mass of graduate cylinder + vinegar Mass of beaker and reaction mixture after reaction Mass of filter paper First mass of dried filter paper and precipitate Second mass of dried filter paper and precipitate Third mass of dried filter paper and precipitate

5 Chemistry 2202 Gravimetric Stoichiometry Lab 2 5 Filtration Technique Items: 1. Draw and label the filtration apparatus you used in this activity. 2. What does "decanting" mean? 3. Why should the liquid from the reaction beaker be decanted into the funnel slowly? 4. Why does the precipitate or residue trapped by a filter have to be rinsed? Analysis 1. What evidence suggests that all of the acetic acid in the vinegar reacted?

6 Chemistry 2202 Gravimetric Stoichiometry Lab Calculate the mass of the Tums tablets before the reaction with vinegar. 3. By subtraction, calculate the mass of unreacted Tums. 4. Calculate the mass of Tums that did react with the vinegar. 5. Using the mass of Tums (calcium carbonate) that did react, use stoichiometry to predict the mass of carbon dioxide that should have been produced (Theoretical Yield). 6. Calcuate the mass of the vinegar that you added to the Tums. 7. Add the mass of the beaker and Tums (before the reaction occurs) to the mass of the viengar determined in question 3. This will give you the total mass of your reactants plus the beaker before the reaction occurs. 8. State the law of conservation of mass.

7 Chemistry 2202 Gravimetric Stoichiometry Lab Compare the total mass of beaker and reactants determined in question 7 to the mass determined after the reaction occurs (Step 6 in Day 1 procedure). Why, if the law of conservation of mass holds, aren't they the same? Account for the difference. 10. Calculate the mass of carbon dioxide actually produced in this reaction from your experimental data. (Hint: See question above!) 11. Calculate the percent yield of carbon dioxide using this equation: actual yield %yield = 100% theoretical yield Error Analysis 12. If you spilled some of the vinegar before you mixed it with the calcium carbonate, how would this affect your predicted yield of carbon dioxide? 13. If you added 4 Tums tablets instead of 3, how would this have affected your predicted yield of carbon dioxide? Explain briefly.

8 Chemistry 2202 Gravimetric Stoichiometry Lab Did you see any bubble of carbon dioxide still submerged in the reaction mixture when you did your final weighing? If so, how would this have affected your predicted yield of carbon dioxide? Explain briefly. Conclusion: